A centriole (green) speeds away from the PCM (red) in a Cnn mutant embryo.

Like a sailboat, centrioles drift away if they aren't properly moored, as Lucas and Raff show on page 725. The researchers pin down a protein that helps keep the structures in place.

A pair of centrioles sits inside a cloud of pericentriolar matrix (PCM), creating the centrosome. It serves as a hub for the microtubules that form the spindle apparatus, which divvies up the chromosomes during mitosis. What connects the centrioles to the PCM and keeps them in position isn't clear. Previous work suggested that the protein centrosomin (Cnn) attracts other proteins to the centrosome. Lucas and Raff wanted to determine whether Cnn tethered the centrioles.

They started with syncytial fly embryos, in which hundreds of dividing nuclei share a common cytoplasm. In embryos lacking Cnn, PCM still gathered at the centrioles, but the centrioles refused to stay put. Instead, they “rocketed” through the cytoplasm, trailing PCM. Microtubules powered the centrioles' escape.

These travels can foul up mitosis because the centriole often loses connection with the spindle poles. Such poles lack asters and can become entangled with neighboring spindles.

Loss of Cnn had a similar effect in larval somatic cells. Centrioles strayed during mitosis, and cells sometimes ended up with extras. The work suggests that Cnn helps tie the centrioles to the PCM. The molecular mechanism is uncertain, but the researchers speculate that Cnn restrains centrioles in the middle of the centrosome by strengthening the PCM.